The present invention relates to an image decoding method, an image decoding apparatus, and a data recording medium and, more particularly to power saving in a digital image decoding process in the monochrome display mode.
In order to store or transmit digital image information efficiently, it is required that digital image information be compressively coded. Currently, there are waveform coding methods such as subband, wavelet, fractal, and so forth, as well as DCT (Discrete Cosine Transform) typical of JPEG (Joint Photographic Coding Experts Group) or MPEG (Moving Picture Experts Group), as a method for compressively coding the digital image information.
Meanwhile, there is a method in which inter-frame prediction is performed using motion compensation by representing values of pixels of a current frame by difference values between these values and values of pixels of a previous frame, and a difference signal of the difference values is subjected to waveform coding, as a method for eliminating redundant image information between adjacent frames or the like.
Hereinafter, a description is given of an image coding method and an image decoding method according to MPEG which employs a DCT process with motion compensation, for explaining a prior art image processing method.
In this image coding method, an input image signal is first divided into plural image signals corresponding to plural blocks (xe2x80x9cmacroblocksxe2x80x9d) into which a display screen is divided, and then the image signals are coded for each macroblock. The xe2x80x9cmacroblockxe2x80x9d as defined herein refers to an image display region comprising 16xc3x9716 pixels on the display screen.
The image signal of each macroblock is divided into image signals corresponding to subblocks which correspond to image display regions each comprising 8xc3x978 pixels, and then the image signals are subjected to a DCT process for each subblock to generate DCT coefficients of each subblock. The DCT coefficients are quantized to generate quantization values for each subblock. This method for coding the image signal by the DCT process and the quantization process is termed xe2x80x9cintra-frame codingxe2x80x9d method.
At a receiving end, the quantization values of each subblock are inversely quantized and subjected to an inverse DCT process to reproduce an image signal corresponding to each subblock.
On the other hand, there is a coding method of an image signal termed xe2x80x9cinter-frame codingxe2x80x9d. In this coding method, a macroblock in which errors between pixels thereof and pixels of a target macroblock to-be-coded are the smallest is detected as a prediction macroblock, in a frame which is temporarily adjacent to a frame to-be-coded, by a method for detecting motion of an image on the display screen such as xe2x80x9cblock matchingxe2x80x9d.
Subsequently, according to the detected image motion, an image signal of a coded frame is subjected to motion compensation, to obtain an optimal image signal for a prediction value of the image signal of the target macroblock. A signal indicating the macroblock (prediction macroblock) with the smallest error is a motion vector. Hereinbelow, a frame including the prediction macroblock that is to be referred to for generating the prediction value is called a xe2x80x9creference framexe2x80x9d.
Thereafter, a difference signal between an image signal of a subblock of the target macroblock and a prediction signal thereof is computed, and then is subjected to the DCT process to generate DCT coefficients, which are quantized to generate quantization values. Then, quantization values for respective subblocks of the target macroblock are transmitted or stored together with the motion information.
At a receiving end, the quantization values (quantized DCT coefficients) are inversely quantized and then subjected to the inverse DCT process to restore the difference signal of each macroblock. Then, an image signal of a decoded reference frame is subjected to motion compensation by the use of the motion vector, to generate the prediction value of an image signal of a target macroblock to-be-decoded. Then, the prediction value and the difference signal are added to reproduce the image signal of the target macroblock.
In this image processing according to MPEG, at a transmitting end, when compressively coding a luminance signal and a color difference signal of a digital image signal, switching between the intra-frame coding and the inter-frame coding is suitably performed for each macroblock, while at a receiving end, switching between the intra-frame decoding and the inter-frame decoding is suitably performed to the compressively coded luminance signal and the compressively coded color difference signal for each macroblock to reproduce the luminance signal and the color difference signal, followed by display of the resulting digital image signal as a color image.
According to the MPEG described above, the image signal is coded in macroblock units each composed of four luminance blocks 701-704 and two color difference blocks 705 and 706, as shown in FIG. 7, and thus coded image signal is transmitted by satellite broadcasting or cable transmission, to be reproduced by an installed receiver or a portable receiver.
In the current situation, power saving, i.e., reduction of power consumed by signal processing, is demanded of the reproducing process of the image signal by the portable receiver.
To be specific, in a case where an image signal is reproduced and displayed as a color image, a coded luminance signal and a coded color difference signal of the image signal are decoded. In this case, for an inter-frame coded image signal, it is necessary to find a prediction value of the color difference signal as well as a prediction value of the luminance signal, which leads to considerable amount of signals processed to find the prediction value, and correspondingly amount of power required for this processing increases.
In color signal display, a reproduced luminance signal xe2x80x9cYxe2x80x9d and reproduced color difference signals xe2x80x9cUxe2x80x9d, and xe2x80x9cVxe2x80x9d must be converted into an RGP signal according to the following equations (1)xcx9c(3):
R=1.164(Yxe2x88x9216)+1.596(Uxe2x88x92128)xe2x80x83xe2x80x83(1)
G=1.164(Yxe2x88x9216)xe2x88x920.813(Uxe2x88x92128)xe2x88x920.391(Vxe2x88x92128)xe2x80x83xe2x80x83(2)
B=1.164(Yxe2x88x9216)+2.018(Vxe2x88x92128)xe2x80x83xe2x80x83(3)
For this conversion, the color difference signals U and V need multiplication, which consumes considerable power.
Consequently, it is difficult to reproduce and display the image signal processed according to MPEG as the color image with saved power, so that the user cannot see a regenerated image for a long period of time.
It is an object of the present invention to provide an image decoding method and image decoding apparatus in which compresively coded image signal can be decoded with power consumption saved, and thereby a regenerated image can be displayed on portable terminal equipment for a long period of time, and a data recording medium which contains an image processing program for implementing decoding by this image decoding method.
Other objects and advantages of the present invention will become apparent from the detailed description given hereinafter. It should be understood, however, that the detailed description and specific embodiment are given by way of illustration only, since various changes and modifications within the scope of the invention will become apparent to those skilled in the art from this detailed description.
According to one aspect of the present invention, in the color display mode, coded data of the luminance signal and the coded data of the color difference signal are decoded, while in the monochrome display mode, coded data of the color difference signal is detected, and the detected coded data of the color difference signal is abandoned and the coded data of the luminance signal is decoded. Therefore, the amount of coded data to be decoded and calculations in RGB conversion can be reduced by about ⅓. As a result, power consumption can be suppressed and the image signal can be reproduced and displayed for a long period of time in the portable terminal equipment.
According to another aspect of the present invention, in the color display mode, the frequency coefficients of the luminance signal is restored to the difference data of the luminance signal and the frequency coefficients of the color difference signal is restored to the difference data of the color difference signal, the motion vector is used to obtain the luminance prediction data, which is added to the difference data of the luminance signal, and the scaled motion vector is used to obtain the color difference prediction data, which is added to the difference data of the color difference signal, to reproduce the luminance signal and the color difference signal, while in the monochrome display mode, the frequency coefficients of the luminance signal are restored to the difference data of the luminance signal, the motion vector is used to obtain the luminance prediction data, which is added to the difference data of the luminance signal to reproduce the luminance signal. Therefore, in the process for decoding the coded data corresponding to the inter-macroblock, the scaling process of the motion vector, the process for generating the prediction data of the color difference block, and the process for adding the prediction data and the difference data of the color difference signal, are dispensed with. As a result, amount of signals to-be-processed is considerably reduced in the decoding process, and correspondingly power consumption is effectively reduced.
According to a further aspect of the present invention, a data recording medium contains a program which makes a computer abandon the coded data of the color difference signal and decode the coded data of the luminance signal in the monochrome display mode. By loading the program into the computer, it becomes possible to implement reproducing process of the compressively coded image signal for a long period of time with saved power.